1. Field of the Invention
This invention relates generally to a separating filter for use with a color television receiver, which separates a luminance signal and a chrominance signal from a composite color video signal, and more particularly relates to a separating filter which can separate a luminance signal and a chrominance signal with no deterioration from a composite color video signal even in the absence of vertical correlativity of the composite color video signal.
2. Description of the Prior Art
To separate a luminance signal and a chrominance signal from a composite color video signal of, for example, NTSC (national television standard committee) system, there has been generally employed a band pass filter in the prior art. But, according to this separating method, all the luminance signals contained within the frequency band of, for example, 3.58 MHz.+-.500 kHz are regarded as the chrominance signal so that the high frequency component of the luminance signal is mixed into the chrominance signal in the portion where the luminance signal changes abruptly resulting in a deterioration of a picture called as cross-color.
While, a so-called comb filter is known, which takes advantage of a vertical correlativity of a picture screen and also the fact that the phase of a subcarrier of the chrominance signal is inverted at every one horizontal period.
FIG. 1 schematically illustrates the circuit arrangement thereof, in which a signal from an input terminal 101 is supplied to an adding circuit 102 and a subtracting circuit 103, and also supplied to the adding circuit 102 and the subtracting circuit 103 by way of a one horizontal period (hereinafter, simply referred to 1H) delay circuit 104. Signals from the adding circuit 102 and the subtracting circuit 103 are supplied through attenuators 105 and 106, each of which attenuates the level of signal supplied thereto to 1/2, and then delivered to output terminals 107 and 108 respectively.
In the circuitry arranged so far, the adding circuit 102 cancels the chrominance signal and then produces the luminance signal at the output terminal 107, while the subtracting circuit 103 cancels the luminance signal and then produces the chrominance signal at the output terminal 108.
But, in this case, if such a signal in which as, for example, shown in FIG. 2A the level of a luminance signal is constant and a chrominance signal is added thereto from the halfway (third scanning line k and in FIG. 2, an arrow represents the level of the luminance signal and a curve represents the chrominance signal) is supplied to the afore-mentioned comb filter, the chrominance signal on the third scanning line k has no vertical correlativity so that the signal is deteriorated. In other words, although at the output terminal 107 is produced the luminance signal as shown in FIG. 2B, the chrominance signal is left on the third scanning line k and a so-called dot interference is caused thereby. Whereas, at the output terminal 108 is produced the chrominance signal as shown in FIG. 2C, but the level of the chrominance signal on the third scanning line k is decreased thus vertical resolution of the chrominance signal being deteriorated.